1. Field of the Invention
This invention relates to an air-conditioning apparatus with an improved operation control system when operating in a dehumidification mode.
2. Description of the Related Art
As an air-conditioning apparatus mounted on, for example an electrical vehicle, in recent years there have been items which employ a refrigeration cycle which doubles in use as a heat pump. Such devices are intended to switch a heater-cooler unit by means of switching, using a valve, a circulation path of refrigerant.
In order to perform dehumidification from a heater-cooler unit with a refrigeration cycle that doubles in use as heat pump, as disclosed in Japanese Patent Application No. 4-107027, which was published Dec. 3, 1993 under No. 5-319077 it has been proposed that a condenser be positioned in an air-conditioning apparatus evaporator within an air duct as well as disposing an exterior heat exchanger outside the air duct. This condenser, exterior heat exchanger, and evaporator are provided in a refrigerant circulation circuit, and by switching a valve provided in this refrigerant circulation circuit and switching the circulation circuit for the refrigerant, the device is changed from a heater to an air-conditioner to a dehumidification device. The operation mode is changed as follows: in the cooling mode, refrigerant discharged from the compressor flows from the exterior heat exchanger to the evaporator and is returned to the compressor, and the exterior heat exchanger functions as an "exterior condenser." In the heater mode, refrigerant discharged from the compressor flows from the condenser to the to the exterior heat exchanger and is returned to the compressor, and the exterior heat exchanger functions as an "exterior evaporator." Further, in the dehumidification mode, refrigerant discharged from the compressor flows from the condenser to the evaporator via the exterior heat exchanger, and air dehumidified and chilled by the evaporator is heated to the target blowing temperature by the condenser and is blown into the passenger compartment.
In order to control the blowing temperature during cooler, heater, or dehumidification modes in such an air-conditioning apparatus, an exterior fan which forces air toward the exterior heat exchanger is provided. The fan varies the heat-exchanging capacity (heat-absorbing or heat-radiating capacity) of the exterior heat exchanger by switching the revolving speed of this exterior fan, thus affecting the heat-radiating capacity of the condenser and the heat-absorbing capacity of the evaporator.
In the case where an air-conditioning apparatus such as that described above is installed in a vehicle such as a van, because of the relationship whereby the exterior heat exchanger is installed horizontally on the bottom surface of the vehicle body, effects due to vehicle-speed wind (wind received in accordance with vehicle travel) with respect to the heat-exchanging capacity of the exterior heat exchanger are hardly exerted. For this reason it is possible to actively control the heat-exchanging capacity of the exterior heat exchanger by means of switching the revolving speed of an exterior fan.
In contrast to this, in the case where the foregoing exterior heat exchanger was installed in a location receiving vehicle speed wind from the front grille, as is the case when mounted on an ordinary vehicle--i.e., a vehicle with a structure that takes in vehicle speed wind from the front grille--the heat-exchanging capacity of the exterior heat exchanger fluctuated in accordance with the magnitude of vehicle speed. Thus, the following problem arose with the prior art.
Briefly, in the cooler mode and the heater mode, the temperature of the evaporator working as a heat exchanger for the cooler and the temperature of the condenser working as a heat exchanger for the heater in the respective modes do not fluctuate greatly even in the case when the heat-exchanging capacity of the exterior heat exchanger has fluctuated, and so there is no large fluctuation in the temperature of the air conditioner air, and no impediment occurs in practical usage.
In the dehumidification mode, however, where the condenser acts as a reheating source for the air dehumidified and chilled by the evaporator, the condenser and exterior heat exchanger function as a refrigerant condenser in a series-connected state, and a phenomenon occurs wherein the temperature of the condenser fluctuates according to the magnitude of the vehicle speed. That is to say, in the case when vehicle speed is large, the heat-radiating performance of the foregoing condenser and exterior heat exchanger, and in its turn the condensation performance, rise and condensation pressure drops, and so the temperature of the condenser experiences a relative decline. Additionally, in the case when vehicle speed is small (including times when the vehicle is stopped), the condensation performance of the condenser and exterior heat exchanger fall and condensation pressure rises, and so the temperature of the condenser experiences a relative rise. Because of this, when in the dehumidification mode the blowing temperature dependent on the temperature of the condenser fluctuates in accordance with the magnitude of the vehicle speed, and there is the problem that comfortable air conditioning cannot be expected.
Additionally, there is the drawback that the adjustment range of the heat-exchanging capacity of the exterior heat exchanger obtained by means of switching the revolving speed of the exterior fan is not sufficient when seen from the temperature adjustment range of the blown air that is actually required, and the temperature adjustment range of the blown air is narrow.